This invention relates to a switching-type power-supply unit that enables the switching with a minimum power loss and a method of switching in the switching-type power-supply unit.
As a conventional switching-type power-supply unit with an output power of about 150 W, a ringing chalk converter, which is a kind of fly-back converter, is often used. An exemplary circuit layout of a ringing chalk converter is shown in FIG. 4. In FIG. 4, the source electric power is supplied to a dotted terminal of the primary winding of the transformer. The other terminal of the primary winding is connected to GND through a switch Qs. By turning on and off the switch Qs at a predetermined timing, electric power is supplied to the secondary winding of the transformer. The output of the secondary winding is rectified and smoothed, and the output voltage of the power-supply unit is generated. A MOSFET is often used as the switch Qs. A capacitor Cr is generally connected in parallel with the MOSFET to construct a “snubber circuit”.
In the switching-type power-supply unit shown in FIG. 4, a zero-current detection circuit determines on-timing of the switch Qs. While, an output voltage of the feedback circuit and a voltage indicating the drain current determines off-timing of the switch Qs.
FIG. 5 shows waveforms of the drain voltage (Vds), the signal output from the control winding to the control unit (Vc), the secondary current (Is) and the drain current (Id).
During the period that the switch is ON, the drain current (Id) is supplied through the inductance Lp of the primary side of the transformer. The drain current (Id) is converted into a voltage by the resistor Rd, and input to the control unit. The control unit compares the voltage with a signal from the feedback circuit connected to the secondary side of the transformer, determines the off timing (t1) of the switch (MOSFET), and turns off the switch. When the switch is turned off, discharge of the energy from the inductance Ls of the secondary side of the transformer starts and a current flows through the diode Dout of the secondary side. The drain voltage (Vds) is kept almost constant during the period from t1 to t2 although it includes some noises as shown in FIG. 5.
When the discharge of the energy from the secondary side of the transformer is completed at t2, the charged electricity in the snubber capacitor Cr starts to discharge. Then, a resonant occurs between Lp and Cr, and the drain voltage (Vds) starts to decrease gradually during the period after t2 as shown in FIG. 5.
The resonant frequency f is determined by Lp and Cr as follows:f=1/(2π*(Lp*Cr)1/2)
The waveform at the control winding of the transformer is shown Vc in FIG. 5. When Vc becomes nearly zero at around t3, which is the timing when the inductance Ls of the secondary side of the transformer completes the discharge. The control unit may turn on the switch Qs and starts the next switching cycle at t3.
However, it is difficult to sense negative voltage by a control unit that is built to operate with positive voltages. Therefore, the detection of zero-point occurs when the voltage Vc is still above zero.
If the switch Qs turns on at t3, as in FIG. 5, the power loss (switching loss), which is calculated as Id*Vds is large because the drain voltage Vds is still high. The power loss is reduced if the switch Qs turns on at the true zero-crossing of Vc at t4 of FIG. 5 or better still when Vc is at a minimum point t5 of FIG. 5.
In order to solve the above mentioned problem, a delay circuit is provided between the control winding of the transformer and the control unit as shown FIG. 4. The delay circuit delays the output signal (Vc) from the control winding to the control unit. This results in that the switch turns on when the drain voltage (Vds) is low enough.
For example, Japanese patent 3,458,369, which is incorporated by reference in its entirety and hereinafter referred as Reference 1, discloses a switching-type power-supply unit including a delay circuit.
However, in the conventional switching-type power-supply unit including a delay circuit:    1) The added delay circuit requires an additional cost.    2) When the capacitance of the snubber capacitor and/or the inductance Lp of the primary side of the transformer are changed, the delay constant changes. Accordingly, a delay circuit with a fixed delay constant cannot be applied for various applications.